Lymph vessels were evaluated in 20 transplanted canine kidneys. Prior to the occurrence of morphological rejection changes, transplants with good blood flow rates show areas with dilated but, less frequently, also undilated lymph vessels. In transplants with the presence of a rejection infiltrate and a decrease of the blood flow rate of 21-50% the lymph vessels may be focally dilated or, in contrast, narrowed by the cells of the rejection infiltrate. Numerous vesicles and vacuoles can be seen within the cytoplasm of endothelial cells. Interendothelial junctions may be occasionally open, the walls of endothelial cells become attenuated, and disintegration of the cell membrane is followed by a focal destruction of the wall and the development of defects. Cells of the rejection infiltrate penetrate through the gaps in the wall of the lymph vessels. The possible mechanism of the disintegration is discussed in this paper.